In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine by a fan, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine by impingement against an airfoil section of the turbine blades, which turns the shaft and provides power to the compressor. The compressor blades and fan blades are usually separate parts that are fretted onto one or more disks attached to a shaft, except in certain situations in which a blisk is used.
In certain aircraft engine designs, a titanium or titanium alloy disk, also referred to as a rotor, has an array of dovetail slots arranged around its outer periphery. Compressor and fan blades, also made of titanium or titanium alloy, have corresponding dovetail bases to allow mate-up of the blade dovetail bases with the respective rotor dovetail slots so that the blade is retained within the dovetail slots. When the rotor is operating at normal operating speeds, centrifugal force causes the blades to move radially outward.
Over time, cracks and failures have been known to develop in the dovetails, due to a variety of causes including aeromechanical response. High cycle fatigue exceeding material capability has been at least one factor in these failures.
Cracks may particularly arise in early stage fan blades, originating in the pressure face of the dovetail, at the contact between the blade and the disk. During engine operation the blade resonates due to engine forced responses and aeromechanical excitations. These vibrations cause sliding and an associated resisting shear force created by the normal force between the blade and disk along with the presence of friction. Areas of local stress concentration are created at the edge of the contact area to the friction that in turn causes a crack to initiate. The stress concentration creates fretting fatigue cracks that propagate to failure due to high cycle fatigue.
One proposed solution to the problem has been a single peening, followed by a Cu—Ni—In wear coating and then to apply a leaded dry film lubricant to the region between the blade dovetail and the rotor dovetail slot, such as that sold under the trade name of MOLYDAG® 254. MOLYDAG® is a federally registered trademark owned by Acheson Colloids Company of Port Huron, Mich. The lubricant lowers the coefficient of friction, thereby decreasing the stress concentration peak value. However, common dry film lubricants include lead, which have been limited or banned as environmentally unfriendly in many countries.
Another proposed solution includes a dual peening followed by an aluminum-bronze wear coating as described in U.S. Pat. No. 6,267,558, assigned to the assignee of the present invention, whose disclosure is incorporated here by reference in its entirety.
What is needed is a lead free surface enhancement for durability for gas turbine engine blade dovetails.